1. Field of the Invention
The present invention relates to ultrasonic motors and, more particularly, to an ultrasonic motor rotated by the utilization of oscillation or vibration which is generated by vibration generating means.
2. Related Art And Prior Art Statement
Conventionally, various technical means have been proposed for improving efficiency of an ultrasonic motor to stabilize driving. For example, as shown in FIG. 11, technical means in which a vibrator 201 which outward ends a1 and a2 having are thickened more than node positions of a driving vibration mode are set whereby the above-described advantages are satisfied is disclosed in Japanese Patent Laid-Open No. HEI 4-91668. That is, the vibrator 201 is capable of reducing the natural frequency or frequency of vibration of the normal mode, which is generally exhibited by a vibrator whose diameter is uniform, by the fact that both the ends a1 and a2 which are further outside than the node position of the vibration are relatively thickened in diameter as compared with a central portion of the vibrator 201. Further, a modal mass is considerably increased to thereby be capable of producing large vibration amplitude.
Moreover, technical means in which both ends of a vibrator, which are located further outside than node positions of a driving vibration mode, are highly densified to produce advantages similar to those of the aforesaid example is also proposed in Japanese Patent Laid-Open No. HEI 4-91670.
However, means in which large-diameter portions are arranged respectively at the node positions of the bending vibration so that an attempt is made to enlarge the vibration amplitude, as indicated in the conventional example is used as is very well known. However, it is generally feared or apprehended that such means results in vibrators having a large size and large mass. If an outer periphery of a portion which becomes a load mass portion increases or is enlarged with respect to an outer peripheral diameter of a piezoelectric element, that is an electric-mechanical energy conversion element, a diameter of the portion is enlarged or large-sized. Furthermore, if the portion which becomes the load mass position is lengthened axially, the whole or entire vibrator is large-sized. In a case where an ultrasonic motor is used at a portion which has sufficient room from the point of view of space, large-sizing of the vibrator does not present a problem. However, in a case where the ultrasonic motor is used in a limited or restricted space, a large-size vibrator is greatly disadvantageous. There is a case where the ultrasonic motor can not be used practically.
Further, in a case of the technical means disclosed in Japanese Patent Laid-Open No. HEI 4-91670, if the high density portion is used at the node positions of the bending vibration, an attempt can be made to miniaturize or down-size the vibrator per se. However, at least two kinds of elements are required. Moreover, there is a fear that such technical means causes an increase in assembly steps and an increase in the cost.